1. Field of the Invention
The present invention relates to a process for the preparation of alcohols by hydrogenating hydroformylation mixtures having high ester contents in the liquid phase, and catalysts for this purpose.
2. Description of the Related Art
Alcohols can be prepared by catalytic hydrogenation of aldehydes which have been obtained, for example, by hydroformylation of olefins. Large amounts of alcohols prepared, for example, in this manner are used as solvents and as intermediates for the preparation of many organic compounds. Important subsequent products of alcohols are plasticizers and detergents.
It is known that aldehydes can be reduced catalytically with hydrogen to give alcohols. Catalysts which contain at least one metal from the groups Ib, IIb, VIa, VIIa, and/or VIIIa of the Periodic Table of the Elements are frequently used for this purpose. The hydrogenation of aldehydes can be carried out continuously or batchwise in the gas or liquid phase using catalysts in the form of powder or pieces.
For the industrial production of alcohols by hydrogenation of aldehydes from the oxo process (hydroformylation of olefins), especially in the case of mass-produced products, continuous processes using catalysts arranged in a fixed bed in the gas or liquid phase are preferred.
In comparison with the gas-phase hydrogenation, the liquid-phase hydrogenation has the more favorable energy balance and the higher space-time yield. With increasing molar mass of the aldehyde to be hydrogenated, i.e. with increasing boiling points, the advantage of the more favorable energy balance increases. Higher aldehydes having more than 7 carbon atoms are accordingly preferably hydrogenated in the liquid phase.
The hydrogenation in the liquid phase has, however, the disadvantage that, owing to the high concentrations of both aldehydes and alcohols, the formation of high boilers due to subsequent and secondary reactions is promoted. Thus, aldehydes can more readily undergo aldol reactions (addition and/or condensation) and form hemiacetals or full acetals with alcohols. With elimination of water or alcohol, the resulting acetals can form enol ethers, which are hydrogenated under the reaction conditions to give the saturated ethers. As a result of formation of these secondary byproducts, the yield is reduced. The byproducts designated as high boilers can at best be partly cleaved in downstream plants to give desired products, such as starting aldehydes and target alcohols.
Industrial aldehyde mixtures which are used for the hydrogenation frequently already contain high boilers in different concentration.
The hydroformylation of olefins in the presence of cobalt and rhodium catalysts gives crude aldehydes which, in addition to aldol products, ethers and acetals, also contain esters, in particular formic esters (formates), as high boilers. If these mixtures are hydrogenated in the gas phase, the major part of the high boilers can be separated off in the evaporator and worked up in a separate process step to give desired products.
In the liquid-phase hydrogenation, on the other hand, the high boilers remain in the reactor feed. They are hydrogenated in the hydrogenation stage for the most part to products from which a desired product can no longer be obtained. Esters present in the reactor feed are frequently not converted into the corresponding target alcohols.
In U.S. Pat. No. 5,059,710, the yield of alcohols obtained by hydrogenation of crude aldehydes is increased by cleaving, in a process stage upstream of the hydrogenation, a part of the high boilers at elevated temperature with water to give aldehydes or alcohols. Hydrolysis and hydrogenation are therefore separate process stages, the document giving no information about the water content of the mixture to be hydrogenated.
A similar process is disclosed in U.S. Pat. No. 4,401,834. Here too, the cleavage of high boilers is effected in the presence of water before the actual hydrogenation step.
GB 2 142 010 claims a process for the hydrogenation of crude aldehydes having 6 to 20 C atoms, which contain high boilers and small amounts of sulfur compounds, to give the corresponding saturated alcohols. The hydrogenation is effected in two reactors connected in series. The first reactor contains an MoS2/C catalyst and the second reactor an Ni/Al2O3 catalyst. The hydrogenation is carried out in both reactors with addition of up to 10% of steam, based on the starting material stream, in the temperature range of from 180 to 260° C. and at a hydrogen partial pressure of from 15 to 21 MPa with a large excess of hydrogen. According to the examples, this is so large that the water added is present virtually only in the gas phase. The object of this process is to suppress the formation of hydrocarbons by hydrogenolysis of the alcohols. No information is given concerning an increase or decrease of high boilers and formates in the hydrogenation.
In U.S. Pat. No. 2,809,220, a liquid-phase hydrogenation of hydroformylation mixtures in the presence of water is described. Sulfur-containing catalysts are used as the catalyst. The hydrogenation is carried out in the pressure range of from 10.5 to 31.5 MPa and in the temperature range of from 204 to 315° C. in the presence of from 1 to 10% of water, based on the starting material. In order to keep the added water in the gas phase, a large excess of hydrogen (from 892 to 3566 Nm3 of hydrogen per m3 of starting material) is used. Regarding the large hydrogen excess, reference is made to the discussion in GB 2 142 010. Furthermore, the high specific energy consumption is disadvantageous in this process.
A further process for the hydrogenation of hydroformylation mixtures is disclosed in DE 198 42 370. This describes how hydroformylation mixtures can be hydrogenated in the liquid phase over copper-, nickel- and chromium-containing supported catalysts. Depending on the process for the preparation of the hydroformylation mixtures (rhodium or cobalt process) these mixtures contain water. The process disclosed is designed for the selective hydrogenation of the aldehydes to alcohols, without hydrogenation of the olefins not converted in the hydroformylation, i.e. the high boilers (especially acetals) are not converted into desired product. This is economically disadvantageous and therefore capable of improvement.
DE 100 62 448 describes a process for the hydrogenation of aldehydes having ester contents of up to 5% by mass to give the corresponding alcohols. The hydrogenation is carried out in the liquid phase in the presence of homogeneously dissolved water. The catalysts used are preferably copper-containing supported catalysts. In this process, aldehydes are hydrogenated to the corresponding alcohols virtually without formation of high boilers. However, there is the disadvantage that the esters are converted only slowly into the corresponding alcohols, so that, with high selectivity, only a low space-time yield results.
DE 35 42 595 describes a two-stage hydrogenation process for the hydrogenation of ester-containing hydroformylation mixtures. There, a supported catalyst comprising silica as a support and nickel and molybdenum as components having hydrogenation activity is used in the first hydrogenation stage, and a catalyst which contains the metals cobalt, copper, manganese and molybdenum having hydrogenation activity is used in the second stage. The main disadvantage of this process is that hydrogenation pressures of 25 MPa are used in order to obtain good yields.